Despite decades of investigation, our understanding of the mechanism for exercise hyperemia remains unsatisfactory. Because of the direct relationship between skeletal muscle blood flow and metabolic activity, it has been theorized that the local metabolic state of the tissue directly affects blood flow by altering vascular smooth muscle tone. However, definitive support for involvement of any specific vasodilator substance is lacking. The working hypothesis underlying this proposal is that the mechanism initiating exercise hyperemia is different than that which sustains the hyperemia and matches blood flow to metabolism. Accordingly, studies to investigate the physiological mechanisms for exercise hyperemia are organized under the following specific aims: Aim 1. To examine the mechanism for vasodilation at the onset of exercise Aim 2. To examine the mechanism for steady-state exercise hyperemia. Provocative results from pilot experiments demonstrate that the immediate increase in blood flow following contraction represents dilation of vascular smooth muscle associated with membrane hyperpolarization. Novel hypotheses to be tested include the concept that mechanical deformation of the vessel wall is responsible for release of a vasoactive agent which initiates vasodilation and the notion that reactive oxygen species may be the link between muscle metabolism and activation of ATP-sensitive potassium channels during steady-state exercise. Since exercise hyperemia represents the coordinated response of a vascular network within skeletal muscle, the ability to perform mechanistic studies in an intact vascular bed represents the major strength of the experimental protocols. This application addresses a topic of fundamental importance which needs novel hypotheses and creative experimental approaches. The results will provide valuable new information regarding the basic physiological mechanisms by which blood flow is regulated to skeletal muscle during dynamic exercise and may have important implications for individuals with peripheral vascular disease.